The present invention relates a low temperature heat shrinkable film and a process for producing the same. More specifically, the present invention relates to a low temperature heat shrinkable film which has high heat shrinkability at low temperature and high shrinkage stress and which has a remarkably excellent transparency as compared with conventional shrink films, and process for producing such a film. Moreover, the present invention relates to an inflation film consisting of a multilayer structure of a single resin.
Conventionally, there have been used low temperature heat shrinkable films exhibiting a low temperature shrinkability at 90.degree. to 100.degree. C. of 30 to 50% which are obtained by drawing a low density polyethylene resin (LDPE) having a long branched chain usually called high-pressure polyethylene, or a blend of the low density polyethylene resin with an ethylene-vinyl acetate copolymer, a butene type rubber or he like at a draw ratio of 1.5 to 3.0.
However, these films are disadvantageous in that, because the LDPE having a number of long branched chains has a remarkably inferior stretchability, it can be drawn only at a low draw ratio and it cannot be uniformly drawn, with the result that unevenness in drawing is generated in the resultant film and unevenness in thickness or jetting are generated in the film due to the internal strain remaining in the film. Therefore, there are serious problems with regard to the productivity of these films. Further, these films have another disadvantage in that they cannot exhibit a display effect to a sufficient degree because the haze value indicative of the transparency of the film, which is a very important factor in a packaging film, is 5% or more, typically about 10%.
The present inventors made extensive investigations to improve the above-mentioned defects. As a result, the present inventors discovered that when a film made of a linear low density polyethylene resin having a short branched chain and density of 0.935 g/cm.sup.3 or less (hereinafter referred to as LLDPE) was drawn at a draw ratio of 3.0 to 6.0 between two rolls having a diameter of 200 mm or less using an air gap of 10 mm or less, a low temperature shrinkable film could be produced which had an extremely excellent transparency, was free from unevenness in drawing, and had a uniform thickness and an excellent film strength.
As is well known, an inflation method is one method for forming plastics films. This inflation method generally comprises blowing air into a tube extruded from an inflation die attached to an extruder to expand the tube, and cooling the expanded tube, thereby continuously forming a plastics film.
When a film is produced by means of an inflation method, in order to prevent the films in the form of a tube from being blocked, the addition of an antiblocking agent is carried out or other measures are adopted.
In the course of extensive investigations to develop a low temperature heat shrinkable polyethylene film having excellent transparency and excellent properties which is suitable for use in shrink packaging, the present inventors discovered that, when a linear low density polyethylene resin having a short branched chain and a density of 0.935 g/cm.sup.3 or less was formed into a film in the form of a tube by means of an air cooling inflation method or a water cooling inflation method, and two films were allowed to be strongly blocked by a strong nip-roll pressure, while effecting sufficient cooling and folding the tubular film by means of a guide plate, in contrast with the conventional technique in which blocking of films is avoided as much as possible, so as to form a two layer structured film, the tubular film could be easily blocked by forming the above-mentioned polyethylene resin according to a water cooling inflation method, a two-stage air cooling inflation method and other methods which exhibit an excellent cooling efficiency because the polyethylene resin has a remarkable tendency to be blocked by a rapid cooling method having a high cooling efficiency in an inflation forming method.
Moreover, it was discovered that when the film obtained above was drawn by means of heat rolls, the blocking of the two films was further strengthened so as to obtain a drawn film of two layer structure which could not be readily separated into two pieces, and at the same time, the transparency was rapidly enhanced to a high level equal to that of a vinylon film (haze value: 3% or less).
In addition, since the above-mentioned drawn film was of a two layer structure in which the two films were blocked, it was found that the film strength such as impact strength and tear strength, especially longitudinal tear strength, was remarkably excellent. As shown in the comparative examples described hereinafter, it was also found that the drawn film exhibited remarkably excellent impact strength and tear strength as compared with an inflation film or a T-die film of single structure having the same thickness. When the strength of a film is to be enhanced, it is a common practice to increase the thickness of the film. In this case, the transparency of the film is likely to be decreased. In contrast, it was found in the above-mentioned case that the transparency became rather improved. As described hereinabove, the conventional low temperature heat shrinkable film was disadvantageous in that the thickness was uneven, the jetting was generated, the quality of the product was poor, and the transparency of the film was unsatisfactory. The present inventors succeeded in obtaining a film of multilayer structure which was heat shrinkable at a lower temperature, had a remarkably excellent transparency and exhibited a conspicuously excellent strength, especially a high resistance to longitudinal tearing, as compared with the conventional inflation films and the like, by press bonding or press bonding and drawing a plurality of films composed of a linear polyethylene resin having a short branched chain and a density of 0.935 g/cm.sup.3 or less by means of an inflation method so as to provide a laminated film.